1. Field of the Invention
The present invention is directed to an inverter circuit for converting DC power and AC power into AC power having arbitrary frequency, a power arm for forming the inverter circuit and a power arm element for forming the power arm.
2. Description of the Background Art
FIG. 8 shows a three-phase inverter circuit as an example of an inverter circuit using a power arm in the background art. In FIG. 8, the reference number V1 represents a DC voltage generating circuit such as a DC voltage source consisting of a commercial AC power supply and a diode bridge circuit or a battery. The reference numbers HB1, HB2 and HB3 represent half bridge circuits including a single-phase power arm for generating an AC voltage from a DC voltage Vcc generated at the DC voltage generating circuit V1.
A power arm of the half bridge circuit HB1 consists of a power arm element including a switching element 1a and a free wheeling diode 2a connected in inverse-parallel connection and another power arm element including a switching element 1b and a free wheeling diode 2b. These power arm elements are connected in series at a node U. An IGBT (insulated gate bipolar transistor), a power bipolar transistor, a power MOSFET (metal oxide semiconductor field effect transistor) and the like are applicable as the switching elements 1a and 1b. 
More particularly, an electrode for inputting current of the switching element 1a (corresponding to a collector when the switching element 1a is an N-channel type IGBT) is connected to a cathode of the free wheeling diode 2a and an electrode for outputting current of the switching element 1a (corresponding to an emitter when the switching element 1a is an N-channel type IGBT) is connected to an anode of the free wheeling diode 2a. The switching element 1b and the free wheeling diode 2b are constituted similarly. The electrode for outputting current of the switching element 1a is connected to an electrode for inputting current of the switching element 1b at the node U.
The electrode for inputting current of the switching element 1a is connected to a terminal of high potential side of the DC voltage generating circuit V1 at a node N1. An electrode for outputting current of the switching element 1b is connected to a terminal of low potential side of the DC voltage generating circuit V1 at a node N2.
Similar to the half bridge circuit HB1, the half bridge circuits HB2 and HB3 respectively include two power arm elements connected in series at nodes V and W. Switching elements of each power arm element are connected to the terminal of high potential side and the terminal of low potential side of the DC voltage generating circuit V1 at the nodes N1 and N2. For brevity, the power arm elements of the half bridge circuits HB2 and HB3 are omitted in FIG. 8.
Three-phase loads (not shown) including three connecting terminals having shapes such as Y shape or xcex94 shape are connected to the nodes U, V and W.
A control signal such as a PWM (pulse width modulation) signal is applied from an HVIC (high voltage integrated circuit) 3 acting as a control circuit to control electrodes (corresponding to a gate when the switching elements 1a and 1b are IGBTs) of the switching elements 1a and 1b of the half bridge circuits HB1. On receipt of the control signal in predetermined timing, each of the switching elements 1a and 1b is turned on and off to generate an AC voltage having arbitrary frequency to be applied to the terminal of the three-phase load connected to the node U. Similar to the half bridge circuit HB1, control electrodes of the switching elements of the half bridges HB2 and HB3 receive a control signal from the HVIC 3 to apply an AC voltage having arbitrary frequency to the terminals of the three-phase loads connected to the nodes V and W.
Peripheral circuits such as a voltage source V2 for operating the HVIC 3, a resistor R1 and a capacitor C1 are connected to the HVIC 3. The HVIC 3 and its peripheral circuits may be individually provided to each of the half bridge circuits HB1, HB2 and HB3. Alternatively, each of the half bridge circuits HB1, HB2 and HB3 can be controlled by a single set of the HVIC 3 and its peripheral circuits. FIG. 8 shows an example in which the HVIC 3 and the capacitor C1 are individually provided to each of the half bridge circuits HB1, HB2 and HB3 and the voltage source V2 and the resistor R1 are shared among the half bridge circuits HB1, HB2 and HIB3.
A ground potential terminal of the HVIC 3 is connected to the node N2 for receiving a ground potential GND from the terminal of low potential side of the DC voltage generating circuit V.
The problems of each of the half bridge circuits of the three-phase inverter circuit shown in FIG. 8 will be described below in reference to FIGS. 9, 10 and 11 taking the half bridge circuit HB1 as an example.
When the switching element 1a is in ON state and the switching element 1b is in OFF state, a current Ic flows from the terminal of high potential side of the DC voltage generating circuit V1 into the terminal of low potential side thereof through the switching element 1a and a three-phase load LD (shown as an inductor in FIG. 9) as shown in FIG. 9.
Next, when the switching element 1a is switched to be in OFF state and the switching element 1b is switched to be in ON state, the flow of the current Ic stops. However, as the current flowing so far is induced to continue flowing by the three-phase load LD, a free wheeling current Ir temporarily flows into the three-phase LD through the free wheeling diode 2b. 
FIG. 10 shows time variation of a potential Vs at the node U considering the ground potential GND as zero. When the switching element 1a is in ON state and the switching element 1b is in OFF state, the potential Vs is approximately the same as the power supply potential Vcc to be provided from the terminal of high potential side of the DC voltage generating circuit V1. However, after time t1 at which the switching element 1a is switched to be in OFF state and the switching element 1b is switched to be in ON state, the flow of the current Ic is suspended until these switching elements are switched again. Due to this, the potential Vs suddenly drops from the power supply potential Vcc.
After transient drop of the potential Vs to a potential Vtr2 having a large degree of a negative value, the potential Vs rises to a potential Vst having a negative value calculated by subtracting a threshold voltage of the free wheeling diode 2b from the ground potential GND and maintains its stationary state until the switching elements are switched again.
When transient increase in a value of the free wheeling current Ir occurs to increase an absolute value of its potential Vtr2 to a certain value or more, the potential Vs at the node U excessively drops. Due to this, increase in a voltage between the control electrode and the electrode for outputting current of the switching element 1a occurs, resulting in the problem of erroneous turn-on of the switching element 1a. As a result of this problem, another problem of malfunction of the HVIC 3 is also caused.
These problems can be avoided by controlling the absolute value of the potential Vtr2 which corresponds to a transient voltage characteristic of the free wheeling diode at a low value. However, in the free wheeling diode of the power arm element in the background art, a thickness of an nxe2x88x92 drift region has been defined to be large to ensure its breakdown voltage having a value approximately the same as that of a breakdown voltage of the switching element. That is, the breakdown voltage of the free wheeling diode, consisting of a cathode electrode CT, n-type semiconductor layers S1a and S1b, a p-type semiconductor layer S2 and an anode electrode AN as shown in FIG. 11, has been ensured by defining a thickness TH of the drift region S1a to be large.
Correlation between the thickness of the nxe2x88x92 drift region and the transient voltage characteristic of the free wheeling diode is positive. Therefore, when the thickness of the nxe2x88x92 drift region is large, the potential Vtr2 during flow of the free wheeling current is likely to have a large degree of a negative value. For this reason, malfunction may occur with high probability in the inverter circuit having the power arm element in the background art.
A first aspect of the present invention is directed to a power semiconductor device, comprising: a switching element having an electrode for inputting current, an electrode for outputting current and a control electrode; and n diodes (nxe2x89xa72) connected in series and each having a cathode an anode, wherein a cathode of a diode arranged at one end of the n diodes is connected to the electrode for inputting current of the switching element and an anode of a diode arranged at another end is connected to the electrode for outputting current of the switching element; and a breakdown voltage between each anode and cathode of the diode is defined to be 1/n of a breakdown voltage between the electrode for inputting current and the electrode for outputting current of the switching element.
According to a second aspect of the present invention, the power semiconductor device according to the first aspect further comprises: n resistors each connected in parallel between respective anodes and cathodes of the n diodes, wherein the n resistors have resistance values approximately equal thereamong.
A third aspect of the present invention is directed to a power arm, comprising: a first power arm element including the power semiconductor device recited in the first or second aspect; and a second power arm element including a switching element having an electrode for inputting current, an electrode for outputting current and a control electrode, and a diode having a cathode connected to the electrode for inputting current and an anode connected to the electrode for outputting current, wherein the first power arm element and the second power arm element are connected in series.
A fourth aspect of the present invention is directed to a power arm, comprising: a first power arm element and a second power arm element each including the power semiconductor device recited in the first or second aspect and connected in series.
A fifth aspect of the present invention is directed to an inverter circuit, comprising: a plurality of power arms, each including a first power arm element including the power semiconductor device recited in the first or second aspect; and a second power arm element including a switching element having an electrode for inputting current, an electrode for outputting current and a control electrode, and a diode having a cathode connected to the electrode for inputting current and an anode connected to the electrode for outputting current, wherein the first power arm element and the second power arm element are connected in series, or including a first power arm element and a second power arm element each including the power semiconductor device recited in the first or second aspect and connected in series; and a control circuit for outputting a control signal that controls each switching element of the power arms, wherein the plurality of power arms are connected in parallel.
A sixth aspect of the present invention is directed to an inverter circuit, comprising: a power arm including a first power arm element including the power semiconductor device recited in the first or second aspect; and a second power arm element including a switching element having an electrode for inputting current, an electrode for outputting current and a control electrode, and a diode having a cathode connected to the electrode for inputting current and an anode connected to the electrode for outputting current, wherein the first power arm element and the second power arm element are connected in series, or including a first power arm element and a second power arm element each including the power semiconductor device recited in the first or second aspect and connected in series; and a control circuit for outputting a control signal that controls each switching element of the power arm.
According to the first aspect of the present invention, as compared with a power semiconductor device in the background art including a single diode having a breakdown voltage approximately the same as that of a switching element and connected in inverse-parallel connection to the switching element, a transient voltage characteristic can be kept low during flow of a free wheeling current into a diode.
According to the second aspect of the present invention, as the resistors having resistance values approximately equal thereamong are respectively connected in parallel to each of the n diodes, voltages generated between an anode and a cathode of each diode are made equal. As a result, transient voltage characteristics of each diode can be uniformed.
According to the third aspect of the present invention, a transient voltage characteristic can be kept low during flow of a free wheeling current into the first power arm element. As a result, there occurs small variation in potential at a point where the first power arm element and the second power arm element are connected in series.
According to the fourth aspect of the present invention, a transient voltage characteristic can be kept low during flow of a free wheeling current into the first and the second power arm elements. As a result, there occurs variation in potential, that is smaller than the variation occurred in the power arm according to the third aspect, at the point where the first power arm element and the second power arm element are connected in series.
According to the fifth aspect of the present invention, there occurs small variation in potential at the point where the first power arm element and the second power arm element are connected in series. As a result, an inverter circuit ensuring a high breakdown voltage and having low probability of malfunction can be obtained.
According to the sixth aspect of the present invention, there occurs small variation in potential at the point where the first power arm element and the second power arm element are connected in series. As a result, an inverter circuit ensuring a high breakdown voltage and having low probability of malfunction can be obtained.
It is therefore an object of the present invention to provide an inverter circuit including a power arm ensuring a high breakdown voltage and having low probability of malfunction.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.